Bioprosthetic heart valves have proven to be successful in the short term, but continue to have poor long-term durability. Even though considerable effort has been put into new heart valve designs, the new valves appear to be only slightly more durable than the old designs. Moreover, their ultimate durability will take another decade to confirm. Part of the problem lies in the lack of a general understanding of what defines the ideal bioprosthetic heart valve. We know that bioprostheses fail in 10 to 20 years while the natural valve can last 8 decades, opening and closing almost 3 billion times. Since the structural matrix of natural valves is not known to repair and remodel, certain inherent structural and functional characteristics must be responsible for the valve's remarkable durability. The broad objectives of this research are to develop a fundamental understandingof valve tissue micromechanics, particularly the viscoelastic effect, so that we can formulate constitutive models of valve tissues that can predict valve stresses under accelerated loading conditions. Such information will be required to carry out well controlled durability tests that can determine the fatigue limit of bioprosthetic heart valves. In the course of this research, we will also obtain material test data that can be used to compare and contrast the mechanics of natural and bioprosthetic heart valves. This information will enable a more proper engineering analysis of existing bioprosthetic heart valves, so the deficiencies in design and materials can be identified and corrected. Data such as this is critical to elevating valve design and testing from an empirical process to a well controlled engineering method. The specific aims of this study are: *Determine the natural preload of the fibrosa and ventricularis, and define the process of load sharing during leaflet elongation of fresh and glutaraldehyde-fixed valves. *Map out the patterns of biaxial strain in fresh and glutaraldehyde-treated porcine aortic valve leaflets. *Develop means of minimizing the variability of materials test data and develop standardized material tests. *Characterize and compare the mechanics of porcine and human aortic valve tissues. *Characterize the viscoelastic response of heart valve tissues and develop a viscoelastic model based on Fung's theory of quasi-linear viscoelasticity. Through a systematic analysis of the material test data that we propose to obtain in this study, we aim to develop a much deeper understanding of aortic valve tissue function, and characterize the deficiencies of existing bioprosthetic tissues. Such information will form the basis for the design of new, more durable, synthetic and bioengineered heart valve substitutes.